Japanese Patent Laying-Open No. 2000-124500 discloses a way of improving light emitting characteristics of a nitride semiconductor laser device, wherein periodic protrusions are formed with a GaN layer stacked on a sapphire substrate, the protrusions are covered flatly with a GaN underlayer, and a gallium nitride-based semiconductor laser device is formed on the GaN underlayer. It also describes that the sapphire substrate may be replaced with a GaN substrate.
Jpn. J. Appl. Phys., Vol. 39, 2000, pp. L647-650 teaches a way of improving an output and a lifetime of a nitride semiconductor laser device, wherein a GaN underlayer is grown to cover a mask pattern of periodic stripe SiO2 masks formed over a main surface of a GaN substrate and windows unprovided with the SiO2 mask, and the nitride semiconductor laser device is formed on the GaN underlayer. According to the article, threading dislocations within the GaN underlayer decrease in a region above the SiO2 mask, and use of the underlayer region containing such less threading dislocations can improve the output and the lifetime of the laser device.
However, it is still desired to improve the lasing lifetime even in the nitride semiconductor laser devices as described above. In view of the foregoing, a major object of the present invention is to provide a nitride semiconductor laser device having a long lasing lifetime.